Anomalous behavior of the high-density excitons and their nonlinear optical response in a quasi-two-dimensional space in a layered crystal BiI 3

Abstract

Spatial and temporal behavior of excitons heavily excited in a quasi-two-dimensional space of a stacking fault interface in BiI 3 has been studied by space- and time-resolved luminescence measurements. The luminescence spectra show the peak-energy shift due to high-density effects. Under the heavier excitation, an anomalous propagation of excitons is observed on the temporal profiles of the luminescence intensity and the peak-shift values at each detecting position. From a comparison with a model calculation, it is seen that transport mechanisms of high-density excitons are basically explained by both diffusion and polariton transport suffering the scattering, while there exist high-density exciton–polaritons which propagate with the polariton group velocity. These results are discussed in terms of a collective motion and coherent transport of excitons.